anon_vma accumulating for certain load still not addressed

[Michal Hocko <mhocko@suse.cz>]

Hi,
back in 2012 [1] there was a discussion about a forking load which
accumulates anon_vmas. There was a trivial test case which triggers this
and can potentially deplete the memory by local user.

We have a report for an older enterprise distribution where nsd is
suffering from this issue most probably (I haven't debugged it throughly
but accumulating anon_vma structs over time sounds like a good enough
fit) and has to be restarted after some time to release the accumulated
anon_vma objects.

There was a patch which tried to work around the issue [2] but I do not
see any follow ups nor any indication that the issue would be addressed
in other way. 

The test program from [1] was running for around 39 mins on my laptop
and here is the result:

$ date +%s; grep anon_vma /proc/slabinfo
1415960225
anon_vma           11664  11900    160   25    1 : tunables    0    0    0 : slabdata    476    476      0

$ ./a # The reproducer

$ date +%s; grep anon_vma /proc/slabinfo
1415962592
anon_vma           34875  34875    160   25    1 : tunables    0    0    0 : slabdata   1395   1395      0

$ killall a
$ date +%s; grep anon_vma /proc/slabinfo
1415962607
anon_vma           11277  12175    160   25    1 : tunables    0    0    0 : slabdata    487    487      0

So we have accumulated 23211 objects over that time period before the
offender was killed which released all of them.

The proposed workaround is kind of ugly but do people have a better idea
than reference counting? If not should we merge it?

---
[1] https://lkml.org/lkml/2012/8/15/765
[2] https://lkml.org/lkml/2013/6/3/568
-- 
Michal Hocko
SUSE Labs

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